In certain types of double seated valves, including ball valves, fluid can be trapped in the cavity enclosed by the seats when the valve is in the closed position. If this fluid is heated, there will be an increase in pressure in this cavity due to fluid expansion. Examples of this phenomenon include a valve used with liquid chlorine at -50.degree. F. being warmed by the sun, or a valve containing liquid hydrocarbons being heated by a fire. If not vented in some way, the consequences of this cavity pressure rise may range from seat distortion resulting in a leaking valve to deformation or destruction of the valve body with a concomitant catastrophic release of the fluid medium to the atmosphere.
One prior art method of solving the problem of cavity pressure build-up involves drilling a hole through the side of the ball so the hole will be pointed in an upstream direction when the ball valve is closed. As the cavity pressure begins to build up, it simply vents into the upstream pipe which is at a lower pressure than the heated cavity. A major drawback of this technique is that the valve becomes unidirectional, i.e. it can only be installed in a line with the vent hole in the ball pointing upstream when the valve is closed. Since this subjects the valve to the risk of improper placement in the field or to inapplicability when the line is to be used to convey fluids in either direction, it has met with less then enthusiastic reception among valve users.